The present invention relates to a method for controlling base station transmitters in a cellular system, the base station comprising at least two transmitters transmitting on at least one control carrier and at least one traffic carrier, wherein the power transmitted on the traffic carrier is, on the average, lower than the power transmitted on the control carrier.
In the GSM radio system (like in any other similar cellular system, such as the PCN system), the channels to be used can be divided into two different classes: traffic channels (TCH) and control channels.
Traffic channels, which transmit either speech or subscriber data, employ a dynamic control of transmission power both at the base station and at the mobile station. The power control word comprises four bits, so that 16 power levels are available. The spacing between two successive power levels is 2 dB, the entire power control range being 30 dB. In the so-called TX-on time slot, in which the transmission takes place, the transmission power (on the traffic channel) is adjusted in each time slot of a frame in accordance with the above-mentioned power control word, to one of the sixteen different power levels.
The control channels are used for signalling and synchronization. The GSM control channels include a broadcast control channel (BCCH) which is a one-way channel from the base station to the mobile stations. The BCCH channel transmits cell-specific data (concerning power levels, frequencies, etc.) to the mobile stations. The power control of the mobile station is based on measurements performed by the base station, and vice versa. Because the measurements performed by the mobile station need a reference level, the power transmitted on the BCCH channel is constant in each cell. At the same time, this constant power is the maximum power of the cell, in order that the signal strength of the channel can be maintained so as to be acceptably strong even within the edge areas of the cell.
At present, the same base station transmitter is used for transmitting the BCCH carrier all of the time, and so this transmitter has to transmit at a power considerably higher than the average transmission power of the base-station transmitters transmitting the traffic carrier. As a result, the mean time between failure (MTBF) of the BCCH transmitter is considerably shorter than that of the transmitters transmitting the traffic carrier, which, in turn, negatively affects the lifetime of the entire transmitter unit of the base station.